Context: GH regulates muscle and fat metabolism, which impacts on body composition and insulin sensitivty, but GH signaling pathways have not been studied in vivo in humans. Objective: We investigated GH signaling in muscle and fat biopsies obtained 30 (n=3) or 60 (n=3) min after an intravenous bolus of GH (0.5 mg) versus saline together with serum sampling in six fasting healthy males. Methods: The expression of the following signal proteins were assayed by Western Blotting: STAT5/pSTAT5, MAPK, and Akt/PKB. IRS1-associated PI 3-kinase activity was measured by in vitro phosphorylation of PI. STAT5 DNA binding activity was assessed with EMSA, and the expression of IGF-I and SOCS mRNA was measured by real-time RT-PCR. Results: GH induced a 52 % increase in circulating FFA levels (P=0.03). Tyrosine phosphorylated STAT5 was detected in muscle and fat of all subjects after GH. Activation of MAPK was observed in several lysates but without GH- dependency. Neither PKB/Akt nor PI 3-kinase activity was affected by GH. GH-induced STAT5 DNA binding and expression of IGF-I mRNA were detected in fat, whereas expression of SOCS1 and SOCS3 tended to increase after GH in muscle and fat. Conclusions: 1) STAT5 is acutely activated in human muscle and fat after a GH bolus, and furtherl downstream GH signaling was significant in fat 2) The direct GH effects in muscle need further characterisation 3) This model may be used to study the mechanisms subserving the in vivo actions of GH on substrate metabolism and insulin sensitivity in muscle and fat.